1. Field of the Invention
The invention relates to an automatic power controller for a laser light source in an optical disk drive, and more particularly to an automatic power controller for the laser light source in an optical disk drive, which is capable of rapidly changing and stabilizing the output power of the laser light source.
2. Description of the Related Art
FIG. 1 illustrates an architecture diagram of a conventional automatic power controller for a laser light source in an optical disk drive. Referring to FIG. 1, the conventional automatic power controller 10 includes a signal source 11, a comparator 12, a drive unit 13, a photo detector 14, and a laser light source (for example, a laser diode) 15. The drive unit 13 receives a comparison signal V2 of the comparator 12 and then generates a drive signal to drive the laser light source 15. The laser light source 15 generates the laser beam with specified power level. The photo detector 14 detects the power of the laser beam emitted from the laser light source 15 and generates a detection signal V3 representing the detected laser output power.
In this architecture, the comparator 12 includes an Operational (OP) amplifier 121, resistors R1 and R2, and a capacitor C. The detection signal V3 is transmitted to a negative input terminal of the OP amplifier 121 via the resistor R1, while a reference signal V1 outputted from the signal source 11 is transmitted to a positive input terminal of the OP amplifier 121. The negative input terminal of the OP amplifier 121 is coupled to the output terminal via the resistor R2 and the capacitor C which are in parallel connection. The architecture diagram of the automatic power controller 10 of FIG. 1 only shows the basic architecture, and an additional amplifier may be disposed between the comparator 12 and the drive unit 13 or between the photo detector 14 and the comparator 12 in the actual design. Moreover, the photo detector 14 could be a front monitor diode (FMD).
Suppose the automatic power controller 10 wants to change the output power of the laser light source 15 from power level Pa to power level Pb, the signal source 11 should change the voltage of the reference signal V1 from V1a to V1b. Under this case, FIGS. 2A to 2D respectively show the detection signal V3 of the photo detector 14, the comparison signal V2 of the OP amplifier 121, and the voltage Vc across the capacitor C. As show in FIG. 2A, the voltage of the reference signal V1 is changed from V1a to V1b. As shown in FIG. 2B, the voltage of the comparison signal V2 of the OP amplifier 121 is changed from V2a to V2c and then to V2b. As shown in FIG. 2C, the voltage of the detection signal V3 of the photo detector 14 is changed from V3a to V3c and then to V3b. As shown in FIG. 2D, the voltage of the voltage Vc across the capacitor C is changed from Vca to Vcb. It is found that as the reference signal V1 being changed, the voltage Vc across the capacitor C is also changed accordingly. As shown in FIG. 2B and FIG. 2D, the comparison signal V2 of the OP amplifier 121 will not reach its steady state until the capacitor C finishes the action of charging/discharging. In addition, in order to achieve better noise immunity of the comparison signal V2 of the OP amplifier 121, the RC constant of the OP amplifier 121 is usually configured to be large enough. However, such a design will then result in that the automatic power controller 10 needs a long time to reach the steady state which is found to frequently cause the servo control in the optical disk drive to fail. For example, the seeking error signal may be out of control range.
FIG. 3 illustrates an architecture diagram of another conventional automatic power controller for a laser light source in an optical disk drive. Referring to FIG. 3, in addition to a second signal source 32, a comparator 12, a drive unit 13, a photo detector 14, and a laser light source (for example, a laser diode) 15, the automatic power controller 30 further includes a first signal source 31, a control unit 33, and two switches SW1 and SW2. The comparator 12 includes an OP amplifier 121, resistors R1 and R2, and a capacitor C. The automatic power controller 30 utilizes the first signal source 31 to generate a control voltage signal, and the control unit 33 to switch the switch SW1 when laser light output power is changed so as to let the control voltage signal to be fed to the drive unit 13 at the beginning of each laser light power changing progress.
Therefore, the drive unit 13 may rapidly generate a correct drive signal to achieve desired output power level of the laser light source 15. Next, the automatic power controller 30 utilizes the switch SW2 to directly output the high voltage (Vcc) or the ground voltage to the OP amplifier 121 at the beginning of each laser light power changing progress so as to make the capacitor C charge/discharge rapidly. Finally, the automatic power controller 30 utilizes the control unit 33 to detect the output voltage of the OP amplifier 121 and the output voltage of the first signal source 31. When both of the output voltages are close to each other, the states of the switches SW1 and SW2 are switched back such that the automatic power controller 30 is switched back to the normal operation mode. Although the automatic power controller 30 may rapidly generate the desired laser output power and stabilize the OP amplifier 121, an extra control unit 33 has to be utilized to detect the output voltage of the OP amplifier 121 and the output voltage of the first signal source 31, and the operations of the switches SW1 and SW2 have to be controlled carefully. For practical implementation, such a design is too complicated.